Implementation of epigraphs using roles

This adds support for epigraphs for quote and verse blocks by using the "epigraph" role.  I couldn't find any tests for the DocBook output of the existing quote and verse blocks, so I added some to make sure I didn't regress anything with my changes.

The quote and verse functions were almost identical, so they were refactored into a single function that did most of the work.  The tests pass after each commit.

Fixes #1195.